A storage arrangement of this kind is known from U.S. Pat. No. 4,954,990. Memories comprising storage cells which comprise a respective MOS transistor with a floating gate are referred to as EPROMs. This means that the individual storage cells can be electrically programmed. Each storage cell contains a bit of a value which depends on whether the floating gate of the storage cell transistor is charged, i.e. programmed, or not. Programming is performed by means of a programming voltage which is applied to given terminals of the transistor of the desired storage cell in order to store a charge on the floating gate.
The value of the programming voltage must be accurately controlled to some extent. When the programming voltage is too high, the storage cell could be destroyed or at least damaged during programming, so that frequent reprogramming is no longer possible. When the programming voltage is too low, the individual storage cells will not be unambiguously programmed, i.e. when they are read it will not be unambiguously clear which value is supposed to be stored. The range of the programming voltage in which neither damaging of the storage cells occurs nor ambiguous programming of the storage cells is possible, however, depends to a given extent on manufacturing tolerances of the integrated memory and is very difficult to determine from outside the memory. Therefore, for the various memories manufactured it is practically impossible to indicate a value of the programming voltage which lies each time within the individual optimum voltage range of the relevant memory.
In the memory arrangement according to the cited U.S. Pat. No. 4,954,990, an additional transistor, having the same properties as the transistors of the storage cells, is used to derive the programming voltage applied to the actual storage matrix from a higher programming voltage or high voltage in such a manner that the programming voltage has a substantially optimum value for the relevant memory. To this end, the current through the additional transistor is measured during application of the high voltage and in dependence on this current the programming voltage is influenced, via a plurality of current mirrors, so that the programming voltage is reduced as the current is larger. This circuit arrangement, however, requires further voltage dividers as well as a large number of components and its operation is not very reliable.